Your Explanation Of The Explosive Sodium-In-Water Reaction Is Incomplete

Many of us gained our first appreciation for chemistry upon witnessing a high school teacher drop a chunk of sodium into a beakerful of water. You may think you know the explanation behind the explosive behavior that follows, but newly published research suggests there's more to the reaction than previously believed.


Top Animation via TREVMANVW

The traditional explanation for the phenomenon goes like this: the reactive sodium (Na) displaces hydrogen atoms from surrounding water (H2O) molecules, generating sodium hydroxide (NaOH) and explosive hydrogen gas (H2). Heat generated from the reaction ignites the hydrogen, creating explosions that raise the temperature and speed the initial reaction even further. But Czech scientists recently demonstrated that the reaction is more wild than anyone might have guessed, and is driven by another force to be reckoned with: electrostatic repulsion.

Illustration for article titled Your Explanation Of The Explosive Sodium-In-Water Reaction Is Incomplete

Flickr user thorius / CC BY-NC-SA 2.0

Reporting their investigation of the deceptively simple Chem 101 trick in Nature Chemistry, chemists in the lab of Pavel Jungwirth instigated the sodium/water explosion under ideal reactive conditions, and used high-speed cameras to capture the ensuing chaos. They used a liquid alloy of sodium and potassium, as well as a surrounding atmosphere of argon gas, to ensure that an explosion would occur every time. A Nature News release has more:

High-speed cameras revealed a vital clue to what was fueling the violent reaction in the early stages. The reaction starts less than a millisecond after the metal droplet, released from a syringe, enters the water. After just 0.4 ms, 'spikes' of metal shoot out from the droplet, too fast to be expelled by heating. What's more, between 0.3 and 0.5 ms, this spiking droplet becomes surrounded by a dark blue/purple colour in the solution.

The reason for these two observations became clear when Jungwirth's colleague Frank Uhlig carried out quantum-mechanical computer simulations of the process with clusters of just 19 sodium atoms. He found that each of the atoms at the surface of the cluster loses an electron within just several picoseconds (10 –12 s), and that these electrons shoot into the surrounding water, where they are solvated (surrounded by water molecules).

Solvated electrons in water are known to have the deep blue colour briefly observed in the videos. Their departure leaves the metal cluster full of positively charged ions, which repel each other. The result is a so-called Coulomb explosion, in which the cluster bursts apart owing to its ions' mutual electrostatic (Coulombic) repulsion.

You should watch the video below to appreciate just how fast this process occurs. The droplet of sodium/potassium metal first expands in response to its initial loss of electrons, creating spikes of metal that extend into the surrounding water. This increases the surface area for the previously-recognized, but slightly slower reaction that generates NaOH and explosive hydrogen gas, and the spectacle snowballs.

Suffice to say, this classic chemistry demo just got way more metal.




The heavier alkali metals give off a bigger boom. We did a demo in school with cesium (?) and holy crap. Always loved throwing heavy alkali metals in water. Makes my inner pyro go for all the squeals.